1. Field of the Invention
This invention relates to a magnetic metal powder that can be used in high-density magnetic recording media, nanoscale electronics, permanent magnet materials, biomolecular labeling agents, drug carriers and the like and to a method of producing the magnetic metal powder. Although, strictly speaking, the magnetic metal powder of the present invention is composed of metal particles characterized by general formula (1) discussed later, owing to the fact that it is typically an FePt-system alloy in which T=Fe, M=Pt, the particles of the magnetic metal powder are referred to in this specification simply by the terms “FePt particles” and “FePt nanoparticles” as being representative of such magnetic metal particles.
2. Background Art
In order to increase the recording density of a high-density magnetic recording medium, it is necessary to reduce the size of the basic unit for recording. However, conventional media using sputter-formed films are approaching the limit to which recording density can be increased because of problems in such areas as thermal fluctuation, grain refining and variance of crystal grain size. Recently, therefore, attention has focused on FePt-system magnetic metal nanoparticles as a high-density magnetic recording medium material that is not susceptible to thermal fluctuation, has high anisotropy, and exhibits strong coercivity.
Japanese Patent No. 3258295 (JPA No. 2000-54012; herein after called Reference 1) teaches a method of producing such magnetic metal nanoparticles, namely a method producing FePt alloy particles in a monodispersed state by conducting thermal decomposition reaction of iron pentacarbonyl simultaneously with reduction of platinum (II) acetylacetonate by the action of polyol. On the other hand, in Journal of Applied Physics, Vol. 87, No. 9, 1 May 2000, p. 5615-5617 (Reference 2), a method of reducing metal ions using boron hydride is reported in which the reaction site is a W/O (water/oil) type reversed micelle utilizing an octane oil phase and CTAB (cetyl trimethyl ammonium bromide) as surface active agent.
The crystal structure of the FePt particles obtained by these methods is a disordered fcc (face-centered cubic) structure, so that the particles exhibit superparamagnetism on the nano order. If they are to be used as ferromagnetic particles, therefore, it is necessary to carry out heat treatment for transforming the crystal structure to an L10 ordered phase that is fct (face-centered tetragonal) phase.
The heat treatment has to be conducted at or above the crystal structure transition temperature (Tt) between the disordered phase and ordered phase and is ordinarily conducted at a high temperature of 450° C. or higher. At the time of this heat treatment, the granularity distribution broadens owing to grain enlargement caused by heat-induced coalescence among the particles. As a result, the particles come to be present in a mixture of single domain and multi-domain structures that makes them unsuitable for a high-density magnetic recording media. Therefore, in order to obtain FePt particles having ferromagnetism while maintaining their grain diameter immediately after synthesis, it is effective to coat the particles with a protective agent for preventing inter-particle coalescence or to lower Tt by some method so that the heat treatment can be conducted at a low temperature.
Denshi Zairyo (Electronic Materials) Jan. 2002, p 61-67 (Reference 3) reports that addition of elements such as Ag, Cu, Sb, Bi and Pb during synthesis of FePt particles by the polyol method makes it possible to lower the crystal structure transition temperature (Tt) between fcc structure and fct structure.
The FePt particles obtained by the methods of References 1-3 have a nonmagnetic fcc (face-centered cubic) structure immediately following the reaction and cannot be used unmodified as magnetic particles for a magnetic recording medium. This makes it necessary to subject them to heat treatment at a temperature equal to or higher than the fct crystal structure transition temperature (Tt) so as to transform them to an fct (face-centered tetragonal) structure exhibiting magnetism.
The crystal structure transition temperature of the FePt particles obtained by these methods is around 450° C., so that heat treatment at a temperature of 450° C. or higher is required for transition to the fct structure. However, when the aggregate (powder) composed of these FePt particles is heated to a temperature of 450° C. or higher, the metal particles enlarge through coalescence. Therefore, even though an fct structure can be obtained, the FePt particles do not assume a nanoparticle morphology suitable for use in a high-density recording medium, and since particle coalescence is usually not uniform, there arises a grain distribution that broadens the range of magnetic characteristic distribution to pose problems from the practical viewpoint.
In order to prevent particle enlargement by heat-induced inter-particle coalescence, the heat treatment must be carried out in a state with the position of the individual particles fixed at prescribed spacing (with the particles fixed at prescribed locations on a substrate, for example). However, realization of such heat treatment requires use of a sophisticated technology for precisely positioning the particles in an orderly fashion. While this may be technically feasible, a still better solution would be for the FePt particles to possess fct structure from immediately after synthesis, because this would offer the considerable merit of eliminating the need for such heat treatment or at least simplify it (by enabling use of a low heat treatment temperature, for example).
Although it has been reported that the Tt temperature is lowered by the effect of elements added to the FePt alloy, the need for heat treatment after reaction still remains, namely a heat treatment temperature of at least 300° C. is required for transition to fct structure, which limits the materials that can be used for the substrate/base and causes a number of other inconveniences. Moreover, in the case where FePt particles of fcc structure are transformed to fct structure by heat treatment conducted after the particles are positioned on a substrate, the particles assume uniaxial crystal magnetic anisotropy during the heat treatment process. The direction of this axis is random when viewed with respect to the substrate, for example. Alignment of the axis in a certain direction with respect to the substrate requires the heat treatment and the like to be carried out in a magnetic field. This is difficult in actual practice. Owing to the fact that particles heat treated on a substrate are adhered to the substrate by sintering or the like, it is extremely difficult to rearrange them as powder on another substrate or base. On the other hand, if the particles should have uniaxial crystal magnetic anisotropy from the start and should further be in the state of a powder enabling the particles to flow freely, it would be easily possible to disperse the particles in resin and uniaxially align them with respect to a substrate at normal temperature by applying a conventional technology used for drying a coating-type magnetic recording medium.